Digital content (hereinafter referred to simply as content), such as images or speech, can be easily copied without undergoing any deterioration. Therefore, currently, additional information, such as control signals for controlling reproduction equipment, data related to the content owners or verification data for preventing data alteration, is being embedded into content to inhibit its copying and alteration. This additional information must be invisibly or inaudibly embedded using a form that does not adversely affect content when that content is employed normally (when an image is displayed or speech is reproduced).
The employment or distribution of moving images as digital data has become popular only relatively recently, but as a result, the protection of copyrights for content having high added value, such as movies, has rapidly grown in importance. Thus, provisions for controlling the copying and reproduction of content have been discussed that include the embedding of the above described additional information. It is presently expected that the protection of DVD (Digital Versatile Disk) content recordings is the practical field for which this control technique will be applied.
For the protection of DVD content, additional information is first embedded into the data for an original image (BaseBand domain). Then, the baseband domain data in which the additional information has been embedded is compressed using MPEG (Moving Picture Experts Group)(MPEG2), and the compressed data is recorded on a DVD and distributed. The data compression that is employed in this instance attenuates the signals that are embedded as additional information into the DVD content.
Generally, when the baseband domain of the content in which the additional information is embedded is compressed using JPEG (Joint Photographic Experts Group) or MPEG, or is converted into digital or analog data, or when this baseband domain of the content is recorded or reproduced by, for example, a home video system, it is highly probable that the signals embedded as additional information will be attenuated. For example, when q denotes the quantized value (hereinafter referred to as a Q value) of predetermined DCT (discrete cosine transform) coefficients used for the compression and x denotes the DCT coefficients after the additional information has been embedded, if |x|<q/2 is satisfied, the value of x is rounded up to 0 and it does not contribute to the detection of the embedded information.
As is described above, when the additional information is embedded into digital content, the signals so embedded are attenuated by data compression, AD/DA conversion, or so. If instead, the additional information can be embedded in accordance with a Q value used for data compression, the robustness of the data compression can be increased. However, it is generally true that the Q value used for compression is not known in advance.
There is a method whereby additional information is embedded into the data of a compressed domain to prevent the deterioration of the additional information due to data compression. However, according to this method, since the information can be embedded only by employing multiple of the Q value used for data compression, the image quality will probably be degraded, thereby rendering this method impractical.